Get the Slips.
While I enjoy the Tena Flex design, the slip style is a better general purpose design. I use the Flex ones like a pullup diaper. They provide protection somewhere between a pull-up and a slip. Without the extra tape to seal the sides, they require extreme caution to avoid flooding. I prefer these over the pull-ups because they are velcroed in place and do not require undressing to change. However, due to the small area for the velcro tabs to attach they are likely to allow the rough velcro tabs to abrade the skin.
The slips are easier to attain and maintain a snug fit around the leg openings which provides much better leak protection. For a diaper which shall be used, the slips are the way to go. When I desire a diaper instead of leak protection, this style is what I grab. The last time I bought Tena diapers, I bought a case of the pull-ups, 3 cases of the Flex, and one case of the slip style. The slip style case was gone within 2 months while I still have almost a full case of the flex over a year later.